The present invention relates to a method, apparatus, program and/or signal for data communication over a channel that is susceptible to interference. Particularly, but not exclusively, the present invention relates to a frame synchronisation technique using forward error correction (FEC).
1. Field of the Invention
FEC techniques have been developed which allow a very low signal energy-to-noise ratio (Es/No). One example is ‘Turbo’ coding as described in ‘Near Shannon Limit error-correcting coding and decoding: Turbo codes’ by C. Berrou, A. Glavieux and P. Thitimajshima, Proc. IEEE ICC, Geneva, May 1993. However, these FEC techniques require the start and end boundaries of the data transmission to be defined precisely by the receiver before the FEC decoding process starts. The detection of the transmission boundaries is known as frame synchronisation or frame sync.
2. Background Art
Frame sync is one of the first operations in the demodulation process; usually, the demodulation process is followed by the FEC process, so that there is a time separation between the frame sync and the FEC process. Within this time separation, frame sync needs to be established successfully with the minimum Es/No required by the FEC process. Known frame sync techniques, as described for example in ‘Optimum Frame Synchronization’, by J. L. Massey, IEEE Trans. Commun., vol. 20, no. 2, Apr. 1972 and ‘Frame Synchronization Techniques’, by R. Scholtz, IEEE Trans. Commun., vol. 28, no. 8, Aug. 1980, achieve frame sync at a low Es/No by means of lengthy Unique Words (UW), specially designed data patterns which are added to each packet transmission. The unique words occupy bandwidth which could otherwise be used to carry user or signalling data.
The use of lengthy unique words is particularly costly of bandwidth in multi-user environments where channel conditions vary over time and per user, such that each burst transmission requires independent acquisition at the receiver. One such environment is a TDMA scheme where it is desirable to use small burst lengths to minimize latency and for efficient resource management; however, this leads to a high ratio of framing overhead to data within bursts.
It would be desirable to find a frame sync technique which improves the data transmission efficiency without sacrificing frame sync quality.
The paper ‘Decoder-assisted Frame Synchronisation for Turbo Coded Systems’, by H. Howlader, Y. Wu and B. Woerner, 2nd International Symposium on Turbo Codes, Brest, France, September 2000, discloses a decoder-assisted frame sync technique in which a synchronisation word is embedded in a packet of data before encoding. The synchronisation word is preceded by a number of flush bits which are necessary to bring the Turbo decoding trellis to a known state. The synchronisation word then guides the decoding trellis through a sequence of known states, if the correct synchronisation has been detected.